1. Field of the Invention
The present invention relates to a total optical conversion circuit for converting NRZ signals to an RZ or soliton format. The present invention also relates to an optical transmission line suitable for transmitting the RZ or soliton signals thus converted. Specifically, this optical transmission line is an optical fiber transmission line in which signals can be transmitted over long distances by selecting an appropriate setting for the dispersion of the transmission lines constituting this optical fiber transmission line.
2. Description of Related Art
It is believed that total optical signal processing circuits will become indispensable for optical communications systems featuring optical fiber transmission lines. In the total optical signal processing circuits currently in the research stage, RZ signals are used as the optical signal format because of good correlation with optical time-division multiplexing. Total optical NRZ/RZ conversion technology is believed to be necessary for future ultra high-speed signal processing.
The following is a description of the general structure of a so-called transponder, which converts an NRZ signal to an RZ signal. As indicated, for example, by Yoneyama et al. in “All-optical clock recovery from NRZ data signal using Mach-Zehnder interferometer with different path length”: 1998 Denshi Johotushin gakkai sogo taikai, Transmission 2, B-10-145 (Literature 1), an NRZ signal is inputted to a delay Mach-Zehnder interferometer and is split in two. The two halves travel along optical paths having mutually different lengths, and are then synthesized again. In the process, an optical signal representing the synthetic sum and an optical signal representing the difference resulting from the two halves canceling each other are outputted from two different output ports of the delay Mach-Zehnder interferometer. Of these signals, the optical signal representing the difference contains the clock component of the inputted optical signal, so this signal is inputted to a mode-locked laser diode whose locking range has the same frequency band, and the RZ clock component is extracted by subjecting this laser to optical injection locking.
In this case, however, only the RZ clock component of the optical signal is extracted, and there is no reproduction of the digital pattern contained in the NRZ optical signal. In addition, a mode-locked laser diode (MLLD) is used in Literature 1 above, but the repetition frequency (frequency locking range) of an MLLD is substantially determined by the resonator length of the laser. With current MLLDs, the desired resonator length is obtained by the cleavage of semiconductor chips, so a mode-locked laser diode suitable for the operating frequency of the system is more difficult to obtain than a regular laser element.
The following is a description of an optical fiber transmission line for transmitting an RZ signal or soliton signal thus generated.
Optical transmission lines used in conventional practice are obtained by connecting, for example, transmission fibers to dispersion-compensated fibers or other components whose wavelength dispersion is opposite in sign to the wavelength dispersion of the transmission fibers. For example, the method disclosed in “4×20 Gbit/s soliton WDM transmission over 2000 km With 100 km dispersion-compensated spans of standard fiber”: Electronics Letters Vol. 33, No. 14, pp. 1234-1235, 3rd Jul. 1997 (Literature 2) is known as a method for stabilizing so-called dispersion-managed solitons propagating along such optical transmission lines. Literature 2 discloses so-called pre-chirp, a technique in which a short optical pulse is provided with a chirp before this pulse is introduced into a transmission line.
In systems such as those described in Literature 2 above, however, optical pulses that are constantly up-chirped in linear fashion propagate toward the outlet of the transmission line. The width of the stable optical pulses at the outlet of the transmission line is therefore greater than the width of freshly formed pulses. Wide optical pulses are unsuitable for obtaining higher bit rates in optical transmission systems. In addition, the necessary amount of pre-chirp in the prechirp technique is determined by the properties of the dispersion-compensated fiber. Applying this method to optical wavelength-division multiplexing transmission causes the chirp amount to vary with the wavelength and makes it difficult to design a transmission line when the dispersion-compensated fiber has a dispersion slope.
A need therefore exists for a simply structured and highly reliable NRZ/RZ converter for converting optical NRZ signals to RZ signals or soliton signals. A need also exists for an optical transmission line through which optical pulses thus generated could propagate in a stable manner.